The preparation by in situ polymerization of monomer reactants or polyimide matrix resins having thermal and oxidative stability and useful for preparing composites is well known. The in situ polymerization method is often referred to as the PMR method and the resulting resin as a PMR resin. In the PMR method, reinforcing fibers or fabric are impregnated with a solution containing a mixture of monomers dissolved in a low boiling alcoholic solvent. The monomers, essentially unreactive at ambient temperatures, polymerize in situ upon evaporation of the solvent and heating of the composite to a suitable reaction temperature. These highly processable polyimide resins are generally highly crosslinked addition-type resins which are suitable for use at temperatures in excess of 250.degree. C.
The preparation of a commonly used PMR polyimide resin is described in U.S. Pat. No. 3,745,149. In such method a dialkyl ester of an aromatic tetracarboxylic acid or corresponding dianhydride, e.g. benzophenone dianhydride, an aromatic diamine and a monoalkyl ester of 5-norbornene-2,3-dicarboxylic acid or its corresponding anhydride, i.e. nadic anhydride, are dissolved in a low boiling point alcoholic solvent and the solution used to impregnate fibers. In situ polymerization of the monomer reactants to form the composite material occurs upon heating the impregnated fiber composite to an elevated temperature. The preparation of such PMR polyimide composites is also described in U.S. Pat. Nos. 4,197,339, 4,244,853 and 4,773,021. See also, Serafini, T. T., Status Review of PMR Polyimides, "Resins for Aerospace", ACS Symposium Series. May Ed., ACS Chapter 2, 15-24 (1980); Serafini, T. T., et al. "A Review of Processable High Temperature Resistant Addition-type Laminating Resins", Applied Polymer Symposium No. 22, 89-100, (1973); and Serafini, T. T., et al., "Thermally Stable Polyimides from Solutions of Monomeric Reactants", 16 J. Appl. Poly. Sci., 905 (1972).
There are, however, several shortcomings associated with such PMR polyimide resin formulations. In order to achieve workable resin flow during processing, relatively low molecular weight resins are necessarily utilized. The resulting cured material is highly crosslinked and tends to be somewhat brittle. Further, low formula molecular weight resins tend to be less thermally stable than the higher molecular weight versions.